1. Field of the Invention
The present invention relates to a color cathode ray tube, and, in particular, to an improvement of the structure wherein a shadow mask of the tube is mounted on a mask frame.
2. Description of the Related Art
FIG. 1 shows a conventional shadow mask type color cathode ray tube. A color cathode ray tube 1 comprises a panel section 2 including a faceplate 4, having a substantially rectangular shape, and a skirt 6 extending from the edge of the faceplate 4, and an envelope 11 including a funnel section 8 connected to the panel section 2 and a neck section 10 continuous to the funnel section 8. A vacuum is created in the cathode ray tube 1 by the panel section 2, funnel section 8 and neck section 10. An electron gun assembly 12 for producing three electron beams 13 is housed in the neck section 10. A deflecting system 14 is arranged on the outside of the cone section of the funnel section 8. The deflecting system 14 produces a magnetic field to deflect the electron beams 13 in a horizontal direction and in a vertical direction. A phosphor screen 16 is formed on the inner surface of faceplate 4 of panel section 2. Within the tube 1, a substantially rectangular shadow mask 18 is arranged so as to face the phosphor screen 16 such that a predetermined gap is produced between the shadow mask 18 and the faceplate 4. The shadow mask 18 is formed of a thin metallic plate and has a number of slit holes 20. A mask frame 22 surrounds the peripheral surface of the shadow mask 18. The mask frame 22 is supported by a plurality of elastic supports 23. A plurality of stud pins 24 engaging the elastic supports 23 are mounted on the inner surface of the skirt 6. The mask frame 22 is provided with an internal magnetic shield 26 for reducing terrestrial magnetic influence upon the electron beams.
The three electron beams 13 emitted from the electron gun 12 are deflected by the deflecting system 14. The deflected electron beams 13 are converged in the vicinity of the slit holes 20 of shadow mask 18. The converged beams 13 are made incident on predetermined areas of phosphor screen 16 where red light, green light and blue light are produced, respectively. Thus, the electron beams 13 produced by electron guns 12 cause the red, green and blue light to be emitted from the phosphor screen 16.
In the color cathode ray tube having the above structure, the internal magnetic shield serves to reduce the terrestrial magnetic influence upon the electron beams. However, in the case where the internal magnetic shield is not provided in the color cathode ray tube, the electron beams emitted from the electron guns are influenced by terrestrial magnetism, and the trajectory thereof are changed. If the trajectory of the electron beams are changed from normal ones, the electron beams do not land on predetermined areas on the phosphor screen. The landing error leads to discoloration on the faceplate and lower picture quality.
The influence by terrestrial magnetism on a color cathode-ray tube will now be described. For example, when the faceplate of the tube is arranged to face northwards in the northern hemisphere, the landing locations of electron beams on the screen are shifted as shown in FIG. 2A. Namely, the landing locations of the beams are shifted on a screen 30 in the direction of arrow 32. On the other hand, when the faceplate is directed southwards, the landing locations of the beams are shifted on the screen 30 by terrestrial magnetism in the direction of arrow 35. This landing error often occurs in the case the internal magnetic shield is not provided.
However, even in the case where the internal magnetic shield is provided in a color cathode ray tube, the following problem may occur. FIG. 3 shows the horizontal lines of magnetic force produced in the tube having the internal magnetic shield (vertical lines of magnetic force are omitted). In this case, the faceplate faces northwards. In this type of tube, the lines of magnetic force produced between the electron gun assembly and the shadow mask are distributed, as indicated by solid lines 36. The lines of magnetic force between the shadow mask and panel section 2 are indicated by solid lines 38. In other words, in the case where the internal magnetic shield 26 is not provided, the lines of magnetic force are distributed as shown by broken lines 40. In contrast, with the provision of shield 26, the lines of magnetic force are changed, as shown by solid lines 36 and 38. The internal magnetic shield 26, shadow mask 18 and mask frame 22, which are formed of magnetic material, constitute a magnetic circuit (a region of low magnetic resistance). As a result, the magnetic flux density in a region within the magnetic circuit (a region through which electron beams pass) is reduced to 1/10. Namely, since the lines 36 of magnetic force are changed through the magnetic members, or the internal magnetic shield 26, shadow mask 18 and mask frame 22, the magnetic flux density in the region where electron beams 13 pass is decreased.
The lines 36 of magnetic force, which have passed through the shadow mask 18, becomes lines of 38 of magnetic force which extend through a vacuum and the face plate 4. The lines 38 of magnetic force are biased toward the tube axis. As a result, the magnetic flux density of lines 38 in the vicinity of the shadow mask 18 becomes higher than that of normal lines 40 of magnetic force. Thus, the electron beams incident on the phosphor screen are intensely influenced, and the problem of a landing error of electron beams remains unsolved.
FIG. 4 shows mask frame 22 disclosed in Japanese Patent Disclosure No. 43242/88, which is designed to reduce the landing error of electron beams. The mask frame has a spring-like metallic support 42 for elastically supporting shadow mask 18. FIG. 5 shows lines of magnetic force produced in the color cathode ray tube in which the support 42 is used. The use of the support 42 prevents the lines of magnetic force from being emitted from the shadow mask 18. In this case, the lines of magnetic force are almost emitted from the mask frame 22. Thus, the magnetic influence on the electron beams produced between the shadow mask and the phosphor screen is reduced, and the landing error of the electron beams is reduced.
However, if the spring-like support 42 is used in a color cathode ray tube used in a recently developed large-sized high-definition television set having an aspect ratio of 16:9, above-described landing error of electron beams in a direction shown in FIG. 2A or FIG. 2B occurs. In this case, when a faceplate of this tube is arranged to face northwards, the landing locations of the beams are shifted in the direction the arrow shown in FIG. 2A. When the faceplate is arranged to face southwards, the landing locations of the beams are shifted in the direction of the arrow shown in FIG. 2B. Namely, the landing locations of the beams are changed so as to circulate over the faceplate. FIG. 6 shows the sum (N/S beam movement amount) of the amount of the landing error in the case of FIG. 2A (the faceplate facing northwards) and the amount of the landing error in the case of FIG. 2B (the faceplate facing southwards). In FIG. 6, the sum of the landing errors at a corner is 98 .mu.m, the sum of the landing errors at an end area on a horizontal axis (X-axis) is 63 .mu.m, and the sum of the landing errors at an end area on a vertical axis (Y-axis) is 33 .mu.m. Substantially no landing error appears at a central area on the faceplate.
In a color cathode ray tube used in a 36-inch high-definition television set having a resolution of 1000, the pitch of each aperture of a shadow mask is 0.39 to 0.48 mm. Thus, as shown in FIG. 7, the diameter .phi.d of each phosphor dot 41 on a phosphor screen is about 170 .mu.m, and the diameter .phi.b of a spot 43 of each electron beam is about 240 .mu.m. Sine a tolerance of a landing error of each electron beam is about 35 .mu.m, an allowance of color purity is very small. Thus, the landing error of the beams, mentioned above, leads to degradation of color purity.
The degradation of color purity can be corrected to some extent by a terrestrial magnetism correction coil 44 shown in FIGS. 8A and 8B. The coil 44 is supplied with a DC current to produce a magnetic field in a direction opposite to the direction of terrestrial magnetism. FIG. 9A shows a landing error of electron beams in the case where an electric current is not caused to flow in the terrestrial magnetism correction coil 44 used in color cathode ray tube of a 36-inch 90.degree.-deflection type high-definition television set. The landing error of electron beams at a corner area is 49 .mu.m, the landing error at an end area on an X-axis is 32 .mu.m, and the landing error at an end area on a Y-axis is 17 .mu.m. FIG. 9B shows the landing error of election beams when the coil 44 is supplied with an electric current to correct the landing error. In FIG. 9B, the landing error at a corner area is zero, but the landing error at the end area on the X-axis becomes 17 .mu.m, and the landing error at the end area on the Y-axis becomes 32 .mu.m. Namely, the correction by the terrestrial magnetism correction coil becomes excessive at the end areas on the X- and Y-axes. Under the circumstance, the correction by the terrestrial magnetism correction coil is not satisfactory.